The c.1261del variant in the glucokinase gene, GCK, causes a frameshift in the protein at codon 421 (NM_000162.5), adding 10 novel amino acids before encountering a stop codon (p.(Glu421SerfsTer10)). While this variant, located in exon 10 of 10, is predicted to cause a premature stop codon and to escape nonsense mediated decay, it is in a functionally important region of a gene where loss-of-function is an established disease mechanism (PVS1). This variant is absent in gnomAD v2.1.1 (PM2_Supporting). This variant was identified in one individual with non-autoimmune or non-absolute/near-absolute insulin-deficient diabetes/hyperglycemia; however, PS4_Moderate cannot be applied because this number is below the ClinGen MDEP threshold (PMID 16965331). However, the clinical history for this individual is highly specific for GCK-MODY (FBG 5.5-8 mmol/L and HbA1c 5.6 - 7.6%) (PP4; PMID: 16965331). Taken together, this evidence supports the classification of this variant as pathogenic for monogenic diabetes. ACMG/AMP criteria applied, as specified by the ClinGen MDEP VCEP (specification version 1.2, approved 6/7/2023: PVS1, PM2_Supporting, PP4.